Secret-chain zero-knowledge proofs and their applications

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1994.Includes bibliographical references (p. 61-64).by Tony Liang Eng.M.S

Subliminal channels in post-quantum digital signature schemes

We analyze the digital signatures schemes submitted to NIST\u27s Post-Quantum Cryptography Standardization Project in search for subliminal channels

Steganography-Free Zero-Knowledge

We revisit the well-studied problem of preventing steganographic communication in multi-party communications. While this is known to be a provably impossible task, we propose a new model that allows circumventing this impossibility. In our model, the parties first publish a single message during an honest non-interactive pre-processing phase and then later interact in an execution phase. We show that in this model, it is indeed possible to prevent any steganographic communication in zero-knowledge protocols. Our solutions rely on standard cryptographic assumptions

Message Transmission with Reverse Firewalls---Secure Communication on Corrupted Machines

Suppose Alice wishes to send a message to Bob privately over an untrusted channel. Cryptographers have developed a whole suite of tools to accomplish this task, with a wide variety of notions of security, setup assumptions, and running times. However, almost all prior work on this topic made a seemingly innocent assumption: that Alice has access to a trusted computer with a proper implementation of the protocol. The Snowden revelations show us that, in fact, powerful adversaries can and will corrupt users\u27 machines in order to compromise their security. And, (presumably) accidental vulnerabilities are regularly found in popular cryptographic software, showing that users cannot even trust implementations that were created honestly. This leads to the following (seemingly absurd) question: ``Can Alice securely send a message to Bob even if she cannot trust her own computer?!\u27\u27 Bellare, Paterson, and Rogaway recently studied this question. They show a strong lower bound that in particular rules out even semantically secure public-key encryption in their model. However, Mironov and Stephens-Davidowitz recently introduced a new framework for solving such problems: reverse firewalls. A secure reverse firewall is a third party that ``sits between Alice and the outside world\u27\u27 and modifies her sent and received messages so that even if the her machine has been corrupted, Alice\u27s security is still guaranteed. We show how to use reverse firewalls to sidestep the impossibility result of Bellare et al., and we achieve strong security guarantees in this extreme setting. Indeed, we find a rich structure of solutions that vary in efficiency, security, and setup assumptions, in close analogy with message transmission in the classical setting. Our strongest and most important result shows a protocol that achieves interactive, concurrent CCA-secure message transmission with a reverse firewall---i.e., CCA-secure message transmission on a possibly compromised machine! Surprisingly, this protocol is quite efficient and simple, requiring only four rounds and a small constant number of public-key operations for each party. It could easily be used in practice. Behind this result is a technical composition theorem that shows how key agreement with a sufficiently secure reverse firewall can be used to construct a message-transmission protocol with its own secure reverse firewall

Subversion-Resilient Signatures: Definitions, Constructions and Applications

We provide a formal treatment of security of digital signatures against subversion attacks (SAs). Our model of subversion generalizes previous work in several directions, and is inspired by the proliferation of software attacks (e.g., malware and buffer overflow attacks), and by the recent revelations of Edward Snowden about intelligence agencies trying to surreptitiously sabotage cryptographic algorithms. The main security requirement we put forward demands that a signature scheme should remain unforgeable even in the presence of an attacker applying SAs (within a certain class of allowed attacks) in a fully-adaptive and continuous fashion. Previous notions---e.g., the notion of security against algorithm-substitution attacks introduced by Bellare et al. (CRYPTO \u2714) for symmetric encryption---were non-adaptive and non-continuous. In this vein, we show both positive and negative results for the goal of constructing subversion-resilient signature schemes. Negative results. As our main negative result, we show that a broad class of randomized signature schemes is unavoidably insecure against SAs, even if using just a single bit of randomness. This improves upon earlier work that was only able to attack schemes with larger randomness space. When designing our new attack we consider undetectability as an explicit adversarial goal, meaning that the end-users (even the ones knowing the signing key) should not be able to detect that the signature scheme was subverted. Positive results. We complement the above negative results by showing that signature schemes with unique signatures are subversion-resilient against all attacks that meet a basic undetectability requirement. A similar result was shown by Bellare et al. for symmetric encryption, who proved the necessity to rely on stateful schemes; in contrast unique signatures are stateless, and in fact they are among the fastest and most established digital signatures available. As our second positive result, we show how to construct subversion-resilient identification schemes from subversion-resilient signature schemes. We finally show that it is possible to devise signature schemes secure against arbitrary tampering with the computation, by making use of an un-tamperable cryptographic reverse firewall (Mironov and Stephens-Davidowitz, EUROCRYPT \u2715), i.e., an algorithm that sanitizes any signature given as input (using only public information). The firewall we design allows to successfully protect so-called re-randomizable signature schemes (which include unique signatures as special case). As an additional contribution, we extend our model to consider multiple users and show implications and separations among the various notions we introduced. While our study is mainly theoretical, due to its strong practical motivation, we believe that our results have important implications in practice and might influence the way digital signature schemes are selected or adopted in standards and protocols